1. Field of the Invention
The present invention relates to a mold for a golf ball and a method of manufacturing a golf ball. More particularly, the present invention relates to an improvement in a mold to be used for forming the cover of a golf ball.
2. Description of the Related Art
A two pieces golf ball is constituted by a core and a cover covering the surface of the core. In the manufacture of the two pieces golf ball, a mold comprising a spherical cavity is used. The mold comprises a support pin capable of being moved forward and backward with respect to the cavity and a vent pin for causing the cavity and outside air to communicate with each other. By the support pin which is moved forward, the core is held on the center of the cavity. Then, a resin composition is put in an injection molding machine and is thus molten, and is injected toward the cavity. The molten resin composition fills a gap between a cavity surface and the core. The molten resin composition covers the core and coagulates to form a cover shortly.
In a stage in which the core is held by the support pin, air is present in the gap between the cavity surface and the core. When the molten resin composition flows into the cavity, the air in the cavity is discharged to an outside. The discharge is carried out through a clearance of the support pin and a clearance of the vent pin.
In some cases, the molten resin composition flows into the clearance to form a fin. The fin is formed along the outer peripheral surface of the support pin or the vent pin. Therefore, the fin is almost ring-shaped. In respect of the appearance of the golf ball, it is necessary to remove the fin. Usually, the fin is removed by grinding. The surface of the golf ball is wholly or partially subjected to the grinding process so that the fin is removed.
In some cases, a large fin cannot be removed by the grinding. The remaining fin considerably deteriorates the appearance of the golf ball. In order to reliably remove the large fin, it is necessary to prolong a time required for the grinding. The grinding for a long time deteriorates the productivity of the golf ball. In addition, if the degree of the grinding is high, precision in the dimension of a dimple formed on the surface of the golf ball is reduced. The reduction in the precision in the dimension adversely affects the aerodynamic characteristics of the golf ball.
If the clearance is reduced, the generation of a large fin can be suppressed. In a mold having a small clearance, however, air is insufficiently discharged in some cases. The insufficient discharge causes defects such as a bare (a space formed by the residual air), the invasion of the air into a cover, scorching (burning caused over the cover) and a weld mark (a linear mark generated in a place in which the resin compositions are bonded to each other). Usually, the resin composition is injected from the equator of a cavity. Therefore, the defects concentrate in the vicinity of a pole in the cavity. In particular, the air is apt to remain just above the pin.
In the case in which the flow speed of the molten resin composition is high, the air is apt to be insufficiently discharged. In recent years, a golf ball comprising a cover having a small thickness has been developed and put on the market. In the cover molding of the golf ball, it is necessary to inject the molten resin composition at a high speed. Consequently, there is a problem in that the discharge of the air is defective.
Japanese Laid-Open Patent Publication No. 2000-37480 has disclosed a technique for forming a support pin by a porous material to enhance the discharging property of air. The support pin causes a hole to be clogged due to repetitive use. The support pin is expensive. In addition, the support pin is fragile and is broken easily. The support pin formed by the porous material is not practical.
Japanese Laid-Open Patent Publication No. 7-80848 and US No. 2003-153410 have disclosed a mold for a golf ball which comprises a slit for discharging air. The structure of the mold is complicated. In the mold, the slit restricts the design of a dimple pattern. The slit is less effective for reducing defects in the vicinity of the pole. The slit is not effective for discharging the air just above a pin. The size of a fin formed by the slit is large. For this reason, the fin damages the appearance of the golf ball.
In a mold attaching importance to the suppression of the fin, the difficulty of the air discharge is generated. On the other hand, in a mold having a clearance increased in order to smoothly discharge the air, a large fin is apt to be generated. The large fin is hard to remove and remains easily. Both the difficulty of the air discharge and the remaining fin deteriorate the quality of a golf ball. It is an object of the present invention to provide a golf ball of high quality.